Article Thermodynamic and Economic Analyses of a New Waste-to- System Incorporated with a -Fired Plant

Peiyuan Pan 1, Meiyan Zhang 1, Gang Xu 1,*, Heng Chen 1,* , Xiaona Song 2 and Tong Liu 1 1 Beijing Key Laboratory of Emission Surveillance and Control for Thermal Power Generation, North China University, Beijing 102206, China; [email protected] (P.P.); [email protected] (M.Z.); [email protected] (T.L.) 2 Electrical and Mechanical Practice Center, Beijing Information & University, Beijing 100192, China; [email protected] * Correspondence: [email protected] (G.X.); [email protected] (H.C.); Tel.: +86-(10)-6177-2472 (G.X.); +86-(10)-6177-2284 (H.C.)  Received: 7 July 2020; Accepted: 20 August 2020; Published: 22 August 2020 

Abstract: A novel design has been developed to improve the waste-to-energy process through the integration with a biomass-fired power plant. In the proposed scheme, the superheated generated by the waste-to-energy is fed into the low-pressure turbine of the biomass power section for power production. Besides, the feedwater from the biomass power section is utilized to warm the air of the waste-to-energy boiler, and the feedwater of the waste-to-energy boiler is offered by the biomass power section. Based on a 35-MW biomass-fired power plant and a 500-t/d waste-to-energy plant, the integrated design was thermodynamically and economically assessed. The results indicate that the net power generated from waste can be enhanced by 0.66 MW due to the proposed solution, and the waste-to- efficiency increases from 20.49% to 22.12%. Moreover, the net present value of the waste-to-energy section is raised by 5.02 million USD, and the dynamic payback period is cut down by 2.81 years. Energy and analyses were conducted to reveal the inherent mechanism of performance enhancement. Besides, a sensitivity investigation was undertaken to examine the performance of the new design under various conditions. The insights gained from this study may be of assistance to the advancement of waste-to-energy technology.

Keywords: waste-to-energy; biomass-fired power generation; steam cycle integration; performance improvement; performance evaluation

1. Introduction Under the stress of energy shortage, global warming, and environmental , the utilization of sources has drawn much attention worldwide [1,2]. Biomass is renewable and can be extensively obtained in numerous forms and types [3]. Biomass include , agricultural crops and residues, animal residues, industrial waste and residues, municipal solid waste (MSW), sewage, etc. [4]. Typically, biomass can be processed in several approaches, such as combustion, gasification, pyrolysis, liquefaction, torrefaction, steam explosion, hydrothermal carbonization, and anaerobic digestion [5]. Final products derived from biomass involve , power, and/or for further usage [6]. Currently, among all thermochemical methods, direct combustion is the most broadly adopted for biomass conversion, which accounts for above 97% of the biomass utilization as energy production globally [4]. Biomass direct combustion is a dominating technology for power generation, and it has great compatibility with traditional thermal power production [7]. It is predicted that biomass

Energies 2020, 13, 4345; doi:10.3390/en13174345 www.mdpi.com/journal/energies Energies 2020, 13, 4345 2 of 20 power will exceed 15% of the overall electricity supply in industrial countries by the end of 2020 [8]. Biomass power generation has a carbon neutralization effect, and regular biomass contains less sulfur and nitrogen than fossil ; thereby, biomass power generation can conduce to the reduction of emissions [9]. Wood and straw are relatively attractive feedstocks for biomass power generation, and agricultural waste products (rice husks, wheat bran, peanut shells, etc.) are usually burned as well [10]. On the other hand, MSW as a biomass source is characterized by higher moisture and volatile matter contents, higher O/C ratio, lower calorific value, poorer grindability, and greater concentrations of alkali or toxic metals in the ashes, as compared to wood/straw, which increases the difficulties in transportation and storage, handling and feeding of materials, flame stability, energy conversion, and plant availability [11]. Hence, the cofiring of MSW and high-quality biomass is hard and probably not beneficial. At present, waste-to-energy (WtE) is a popular method for , due to its features of dramatical MSW reduction and affordable costs for heat/electricity generation, especially for developing countries [12]. Incineration will handle more than half of the MSW produced in China by the end of 2020, based on the Chinese 13th Five-Year Plan (2016–2020) [13]. Nevertheless, the waste-to-electricity efficiencies of conventional WtE incineration plants are relatively low, ranging from 14% to 28% [14], and the poor performance of a WtE plant is mainly caused by the limited steam parameters, small capacity, and simple steam cycle [15]. Much research has been devoted to improving the WtE process, chiefly including promoting steam parameters, decreasing heat loss, and a combination with other thermal systems. MSW contains high contents of Cl, S, Na, K, and other alkali metals, which can form HCl, sulfate, alkali metal salts, and other pollutants during the incineration, probably inducing severe corrosion and fouling in the boiler [16]. Thus, the live steam parameters of a WtE boiler are seriously restricted and generally around 4 MPa and 400 ◦C. Xu et al. [17] proposed a solution that uses phase-change materials-based refractory bricks on the wall of a WtE boiler to raise the steam temperature. Bogalea et al. [18] described a design that divides the flue gas into two fractions, and the lower corrosive part is delivered to a separate superheater to enhance the steam parameters. Another approach to promote the steam temperature is employing a radiant superheater in the WtE boiler [19]. Besides, the reduction of the exhaust gas heat loss can be achieved through flue gas recirculation [20]; diminishing excess air [21]; and recovery, such as heating combustion air/feedwater [22], producing district heat [23], and driving an organic Rankine cycle [24]. Concerning system integration, Consonni et al. [25] and Poma et al. [26] discussed a hybrid WtE-gas turbine combined cycle scheme, where the steam yielded from the WtE boiler is further heated by the gas turbine exhaust gas and then enters into the for power production. Mendecka et al. [27] presented a novel WtE design incorporated with a concentrating system, in which the is harvested to superheat the live steam of the WtE boiler. Chen et al. [28] explored the integration of a WtE plant and a -fired power plant by feeding the heat acquired from the incineration into the steam cycle of the coal power section. However, little has been published on integrating a WtE plant with a biomass-fired power plant based on the steam cycle incorporation, which may contribute to improving the waste-to-electricity efficiency. This paper proposed an innovative WtE design, which is combined with a biomass-fired power plant based on the steam cycle. In the integrated scheme, the superheated steam exported from the WtE section enters into the turbine of the biomass power section for producing electricity. Meanwhile, the combustion air of the WtE boiler absorbs energy from the feedwater extracted from the biomass power section and the saturated steam taken from the WtE boiler, and the biomass power section supplies feedwater for the WtE boiler. Consequently, the waste-to-electricity efficiency may gain significant improvement, and some equipment of the WtE plant can be saved, resulting in more revenues and lower costs. To check the feasibility of the novel concept, thermodynamic and economic evaluations were undertaken based on a 500-t/d WtE plant and a 35-MW biomass-fired power plant. Energy and exergy analyses were performed to investigate the root cause of efficiency enhancement. Additionally, the influence of the boiler loads on the performance of the new design was examined. Energies 2020, 13, x FOR PEER REVIEW 3 of 20

Energy and exergy analyses were performed to investigate the root cause of efficiency enhancement. Energies 2020, 13, 4345 3 of 20 Additionally, the influence of the boiler loads on the performance of the new design was examined.

2.2. ReferenceReference PlantsPlants andand ConceptConcept ProposalProposal

2.1.2.1. ReferenceReference WtEWtE PlantPlant AA typicaltypical WtEWtE plantplant withwith aa dailydaily disposaldisposal capacitycapacity ofof 500500 tt hashas beenbeen selectedselected forfor thethe casecase study,study, whichwhich isis sketchedsketched inin FigureFigure1 .1. The The feedwater feedwater enters enters into into the the WtE WtE boiler boiler as as a a working working fluid fluid and and is is heatedheated byby thethe flueflue gasgas generated generated fromfrom the the waste waste incineration. incineration. AfterAfter absorbingabsorbing thethe heatheat energyenergy inin thethe boiler,boiler, thethe yieldedyielded steamsteam flowsflows intointo the the turbine turbine for for power power production. production. TheThe exhaustexhaust gasgas ofof thethe WtEWtE boilerboiler isis dischargeddischarged afterafter thethe treatmenttreatment ofof thethe flueflue gasgas scrubberscrubber (FGS)(FGS) andand thethe bagbag filterfilter (BF).(BF). TableTable1 1 providesprovides the the basic basic data data of of the the WtE WtE plant. plant. TheThe parametersparameters ofof thethe superheatedsuperheatedsteam steamat at thethe turbineturbine inletinlet areare merelymerely 3.903.90 MPaMPa andand 395.0395.0◦ °C,C, andand thethe waste-to-electricitywaste-to-electricity eefficiencyfficiency ofof thethe plantplant cancan onlyonly getget 20.49%.20.49%. Besides,Besides, thethe feedwaterfeedwater intointo thethe WtEWtE boilerboiler isis preheatedpreheated byby twotwo regenerativeregenerative heatersheaters (RHs),(RHs), thethe deaerator deaerator (DEA) (DEA) and and RH2, RH2, and and their their parameters parameters are are given given in in Table Table2. 2.

  Generator

Turbine Drum   SAH Condenser  RH2

 WtE boiler DEA CP PAH3 (RH1) FWP

 PAH2  BF Stack FGS  PAH1

BF: bag filter DEA: deaerator CP: condensate pump FGS: scrubber FWP: feedwater pump PAH: primary air heater RH: regenerative heater SAH: secondary air heater WtE: waste-to-energy

Figure 1. Reference WtE plant. Figure 1. Reference WtE plant. Table 1. Basic parameters of the reference waste-to-energy (WtE) plant. MSW: municipal solid waste. Table 1. Basic parameters of the reference waste-to-energy (WtE) plant. MSW: municipal solid Itemwaste. Unit Value Feedstock (MSW) flow rate t/h 20.84 Item Unit Value Lower heating value MJ/kg 7.00 Feedstock (MSW) flow rate t/h 20.84 Fuel energy input MW 40.51 Lower heating value MJ/kg 7.00 Flow rate t/h 48.60 LiveFuel steam energy input MW 40.51 Pressure MPa 3.90 (into turbine) Flow rateTemperature C t/h 395.0 48.60 Live steam$$ ◦ Pressure MPa 3.90 (into turbine) Flow rate t/h 35.75 Exhaust steam TemperaturePressure kPa°C 6.80 395.0 (into condenser) Flow rateTemperature ◦C t/h 38.5 35.75 Exhaust steam$$ Exhaust gas temperaturePressure (out of boiler) ◦CkPa 190.0 6.80 (into condenser) Boiler capacity (heat absorbedTemperature by working fluid) MW°C 37.47 38.5 Exhaust gas temperatureBoiler e ffi(outciency of boiler) % °C 78.53 190.0 Net power output MW 8.30 Boiler capacity (heat absorbed by working fluid) MW 37.47 Waste-to-electricity efficiency % 20.49

Energies 2020, 13, 4345 4 of 20

Table 2. Parameters of the heat regeneration system in the reference WtE plant. DEA: deaerator and RH: regenerative heater.

Item DEA (RH1) RH2 Flow rate (t/h) 3.49 3.74 Extraction steam Pressure (MPa) 0.27 0.08

Temperature (◦C) 195.4 92.8 Flow rate (t/h) - 3.74 Drain water Temperature (◦C) - 92.8 Outlet flow rate (t/h) 51.1 39.74 Feedwater Outlet temperature (◦C) 130.0 88.0

The essential combustion air is necessary to be preheated for the stable combustion in the boiler furnace. Table3 shows the data of the air preheating system in the reference WtE plant. The primary air is heated by the steam extraction of the turbine and the steam from the drum in the three-stage primary air heater (PAH), and its temperature is eventually raised to 220.0 ◦C. The secondary air is warmed from 15.0 ◦C to 166.0 ◦C by the steam extraction from the turbine in the secondary air heater (SAH).

Table 3. Parameters of the air preheating system in the reference WtE plant. PAH: primary air heater and SAH: secondary air heater.

PAH Item SAH PAH1 PAH2 PAH3 Inlet pressure (MPa) 4.54 1.31 4.54 1.31 Hot fluid Inlet flow rate (t/h) 2.23 3.89 2.23 1.76 (steam/water) Inlet/outlet temperature (◦C) 225.3/104.3 287.3/98.3 258.0/225.3 287.3/98.5 Inlet flow rate (t/h) 73.84 73.84 73.84 30.17 Cold fluid (air) Inlet/outlet temperature (◦C) 15.0/31.0 31.0/166.0 166.0/220.0 15.0/166.0

2.2. Reference Biomass-Fired Power Plant Figure2 displays the diagram of the reference biomass-fired power plant, which mainly involves a vibrating grate boiler, an extraction turbine, and a generator. The feedstock into the biomass power plant is primarily stalk. Table4 presents the fundamental data of the biomass power plant, and the parameters of the live steam into the turbine can reach 535.0 ◦C and 9.40 MPa. In the heat regeneration system, the feedwater out of the condensate pump (CP) passes through RH6-1 successively to get its design temperature, and the data of the heat regeneration system is collected in Table5. The biomass power plant runs much more efficiently than the WtE plant, and its thermal efficiency can attain 30.13%.

2.3. Proposed Hybrid System A novel WtE design has been put forward to improve the energy utilization of the MSW, as depicted in Figure3. Several connections between the WtE section and the biomass power section have been established, aiming at the steam cycle. In the new scheme, the steam turbine of the biomass power plant is divided into two parts, the high-pressure turbine (HPT) and the low-pressure turbine (LPT). The superheated steam derived from the WtE boiler is sent to the LPT for power production, and the biomass power section provides feedwater for the WtE boiler. Moreover, a three-stage PAH and a double-stage SAH are arranged to accomplish the air preheating for the WtE boiler. Partial feedwater obtained from the RH6 outlet is taken to the PAH1 and SAH1 to warm the air; afterward, the feedwater extracted from the DEA outlet is transferred to the PAH2 and SAH2 to raise the air temperature. The primary air is further heated to 220.0 ◦C in the PAH3 by the saturated steam of the WtE boiler, Energies 2020, 13, 4345 5 of 20 and then, the condensate out of the PAH3 is delivered to the DEA inlet after decreasing its pressure by the pressure reducing valve (PRV). The condensate out of the PAH1 and SAH1 is pumped into the RH6 by the additional pump (AP), and the condensate of the PAH2 and SAH2 is supplied to the WtE boiler as feedwater. The exhaust gas of the WtE boiler is still treated by the FGS and BF and is finally expelled via the stack of the biomass power plant. Attributed to the proposed solution, the WtE section and biomass power section exploit the same steam turbine, and the overall power production could be improved. Besides, some components (turbine, RHs, condenser, generator, stack, etc.) of the WtE plant are not needed after the hybridization. Above all, the suggested integration may contribute to a remarkable increment in the waste-to-electricity efficiency, and the capital costs of the WtE system couldEnergies be 2020 dramatically, 13, x FOR PEER cut REVIEW down. 5 of 20

Generator

Turbine Drum

Condenser

DEA (RH3) RH1 RH2 RH4 RH5 RH6 CP Biomass-fired boiler

FWP FGD Stack

BF FGD: flue gas desulfurization

FigureFigure 2.2. Reference biomass-firedbiomass-fired powerpower plant.plant.

Table 4. Basic parameters of the reference biomass-fired power plant. Table 4. Basic parameters of the reference biomass-fired power plant. Item Unit Value Item Unit Value Feedstock (biomass) flow rate t/h 39.13 Feedstock (biomass) flow rate t/h 39.13 Lower heating value MJ/kg 9.435 Lower heating value MJ/kg 9.435 Fuel energy input MW 102.55 Fuel energy input MW 102.55 Flow rate t/h 128.99 Live steam Flow rate t/h 128.99 Pressure MPa 9.40 Live steam$$ (into turbine) Pressure MPa 9.40 (into turbine) Temperature ◦C 535.0 TemperatureFlow rate t/h°C 91.80 535.0 Exhaust steam Flow ratePressure kPa t/h 4.90 91.80 Exhaust (into condenser) steam$$ PressureTemperature ◦CkPa 32.5 4.90 (into condenser) Exhaust gas temperatureTemperature (out of boiler) ◦C°C 127.0 32.5 ExhaustBoiler gas capacity temperature (heat absorbed (out of by boiler) working fluid) MW °C 91.36 127.0 Boiler capacity (heat absorbedBoiler effi byciency working fluid) % MW 89.10 91.36 Net power output MW 30.90 Boiler efficiency % 89.10 Biomass-to-electricity efficiency % 30.13 Net power output MW 30.90 Biomass-to-electricity efficiency % 30.13

Table 5. Parameters of the heat regeneration system in the reference biomass-fired power plant.

DEA Item RH1 RH2 RH4 RH5 RH6 (RH3) Flow rate (t/h) 7.88 5.83 3.64 5.04 5.80 9.00 Extraction Pressure (MPa) 2.55 1.32 0.59 0.40 0.19 0.07 steam Temperature (°C) 381.4 353.5 278.2 190.5 126.8 88.2 Flow rate (t/h) 7.88 13.72 - 5.04 10.84 19.84 Drain water Temperature (°C) 192.4 158 - 118.4 88.2 32.5 Feedwater Outlet flow rate (t/h) 128.99 128.99 128.99 111.67 111.67 111.67

Energies 2020, 13, 4345 6 of 20

Table 5. Parameters of the heat regeneration system in the reference biomass-fired power plant.

Item RH1 RH2 DEA (RH3) RH4 RH5 RH6 Flow rate (t/h) 7.88 5.83 3.64 5.04 5.80 9.00 Extraction steam Pressure (MPa) 2.55 1.32 0.59 0.40 0.19 0.07

Temperature (◦C) 381.4 353.5 278.2 190.5 126.8 88.2 Flow rate (t/h) 7.88 13.72 - 5.04 10.84 19.84 Drain water Temperature (◦C) 192.4 158 - 118.4 88.2 32.5 Outlet flow rate (t/h) 128.99 128.99 128.99 111.67 111.67 111.67 Feedwater Outlet temperature ( C) 220.0 187.4 158.0 139.3 114.4 84.2 Energies 2020, 13, x FOR PEER REVIEW ◦ 7 of 20

Generator

HPT LPT Drum1

Condenser

DEA (RH3) CP RH1 RH2 RH4 RH5 RH6

Biomass- fired boiler AP

  FWP1  FGD Stack

BF1

PRV   

Drum2  SAH2 SAH1   

FWP2 PAH3 PAH2 PAH1 WtE boiler BF2

FGS

AP: additional pump HPT: high-pressure turbine LPT: low-pressure turbine PRV: pressure reducing valve

Figure 3. Proposed WtE system integrated with a biomass-fired power plant. Figure 3. Proposed WtE system integrated with a biomass-fired power plant.

Table 7. Comparison of the simulation results and design data of the reference biomass-fired power plant.

Relative Item Design Calculated Error (%) Feedstock (biomass) flow rate (t/h) 39.13 39.13 0.00 Flow rate (t/h) 128.99 128.99 0.00 Live steam$$ Pressure (MPa) 9.40 9.40 0.00 (into turbine) Temperature (°C) 535.0 535.0 0.00 Flow rate (t/h) 91.69 91.80 +0.12 Exhaust steam$$ Pressure (MPa) 4.90 4.90 0.00 (into condenser) Temperature (°C) 32.5 32.5 0.00 Exhaust gas temperature (out of boiler) (°C) 127.0 127.0 0.00

Energies 2020, 13, 4345 7 of 20

3. System Simulation To assess the performance of the integrated scheme as compared to that of the separate scheme (including the conventional WtE plant and biomass-fired power plant), the software EBSILON Professional (Version 13.0; STEAG Energy Services GmbH) was employed to carry out the simulation/calculation. EBSILON Professional has an uninterrupted model structure with standard components that are used for modeling various thermal systems, and it can balance a model with the help of a closed solution system (based on a sequential solution method) [29]. In this software, the reference biomass power plant, the reference WtE plant, and the hybrid system were modeled using the built-in components. The built models were validated by comparing the calculation results with the design data of the reference plants, and some of the comparisons are presented in Tables6 and7. As the relative errors are very small, the simulation models seem to be accurate and available.

Table 6. Comparison of the simulation results and design data of the reference WtE plant.

Item Design Calculated Relative Error (%) Feedstock (MSW) flow rate (t/h) 20.84 20.84 0.00 Flow rate (t/h) 48.60 48.60 0.00 Live steam Pressure (MPa) 3.90 3.90 0.00 (into turbine) Temperature (◦C) 395.0 395.0 0.00 Flow rate (t/h) 35.68 35.75 +0.20 Exhaust steam Pressure (MPa) 6.80 6.80 0.00 (into condenser) Temperature (◦C) 38.5 38.5 0.00

Exhaust gas temperature (out of boiler) (◦C) 190.0 190.0 0.00 Net power output (MW) 8.31 8.30 0.12 − Waste-to-electricity efficiency (%) 20.51 20.49 0.10 −

Table7. Comparison of the simulation results and design data of the reference biomass-fired power plant.

Item Design Calculated Relative Error (%) Feedstock (biomass) flow rate (t/h) 39.13 39.13 0.00 Flow rate (t/h) 128.99 128.99 0.00 Live steam Pressure (MPa) 9.40 9.40 0.00 (into turbine) Temperature (◦C) 535.0 535.0 0.00 Flow rate (t/h) 91.69 91.80 +0.12 Exhaust steam Pressure (MPa) 4.90 4.90 0.00 (into condenser) Temperature (◦C) 32.5 32.5 0.00

Exhaust gas temperature (out of boiler) (◦C) 127.0 127.0 0.00 Net power output (MW) 30.95 30.90 0.16 − Biomass-to-electricity efficiency (%) 30.18 30.13 0.17 −

4. Thermodynamic Analysis

4.1. Basic Hypotheses For comparing the performances of the two schemes, a few crucial hypotheses have been brought forward: Energies 2020, 13, 4345 8 of 20

(a) The feedstock flow rates of the biomass power plant and the WtE plant are fixed. (b) The exhaust gas temperatures and efficiencies of the WtE boiler and the biomass-fired boiler remain unchanged. (c) The biomass-to-electricity efficiency and power output of the biomass power plant are invariable. (d) The temperature and pressure of the dead state are 288.15 K and 101.325 kPa. (e) The surroundings’ effects are neglected.

4.2. Energy Analysis Through the simulation of EBSILON Professional, the parameters of the proposed hybrid system were determined considering the design data of the reference WtE plant and biomass power plant. Numerous groups of parameters were derived for the hybrid system during the calculations, and the optimal group has been selected and presented in this paper. In the new design, the feedwater from the heat regeneration system of the biomass power section and the saturated steam from the WtE boiler are utilized to warm the air fed into the WtE boiler. Table8 shows the parameters of the air preheating process in the WtE section of the integrated scheme. In PAH1 and SAH1, the temperatures of the primary air and the secondary air are increased to 74.1 ◦C by absorbing heat from the feedwater fetched from the RH6 outlet. Then, their temperatures augment to 166.0 ◦C through the heating in the PAH2 and SAH2. Furthermore, the hot primary air is heated to 220.0 ◦C by the saturated steam provided by the WtE boiler.

Table 8. Parameters of the air preheating system in the WtE section of the integrated scheme.

PAH SAH Item PAH1 PAH2 PAH3 SAH1 SAH2 Inlet pressure (MPa) 0.89 0.87 4.54 0.89 0.87 Hot fluid Inlet flow rate (t/h) 18.04 36.29 2.23 7.38 14.83 (water/steam) Inlet/outlet temperature (◦C) 93.2/35.0 174.0/130.1 258.0/225.3 93.2/35.0 174.0/130.1 Inlet flow rate (t/h) 73.84 73.84 73.84 30.17 30.17 Cold fluid (air) Inlet/outlet temperature (◦C) 15.0/74.1 74.1/166.0 166.0/220.0 15.0/74.1 74.1/166.0

The waste-to-electricity efficiency (ηen,w) and the total energy efficiency (ηen,tot) have been defined to measure the energy performances of the studied systems.

Pw ηen,w = (1) (mw/3.6) qw,net ×

Ptot ηen,tot = (2) (m /3.6) q + (mw/3.6) qw,net b × b,net × where Ptot and Pw are the net total power output and the net power output of MSW, kW, qb,net and qw,net are the lower heating values of the biomass and MSW, kJ/kg, mb and mw are the feedstock flow rates of the biomass-fired boiler and WtE boiler, t/h. The net power output of the biomass remains invariable after the integration; thereby, the power generated by the WtE section in the integrated scheme can be determined as:

Pw = Ptot P (3) − b where Pb is the net power output of biomass, kW. The thermal performances of the two schemes are presented in Table9. As the feedstock (biomass and MSW) flow rates in the two schemes are maintained constant, the gross total power production of the integrated scheme is 0.47 MW larger than that of the separate one. Due to the removal of several devices (for instance, the circulating water pump and CP), the total parasitic power consumption Energies 2020, 13, 4345 9 of 20 declines from 4.90 MW to 4.71 MW. Hence, the net total power output is improved from 39.20 MW to 39.86 MW, and the total energy efficiency grows from 27.40% to 27.86%. Under the condition that the net power output of biomass is considered as unchanged after the hybridization, the waste-to-electricity efficiency increases by 1.63 percentage points.

Table 9. Thermal performances of the two schemes.

Item Unit Separate Scheme Integrated Scheme Difference Feedstock (biomass) flow rate t/h 39.13 39.13 0.00 Feedstock (MSW) flow rate t/h 20.84 20.84 0.00 Gross total power output MW 44.10 44.57 +0.47 Total auxiliary power MW 4.90 4.71 0.19 − Net total power output MW 39.20 39.86 +0.66 Net power output of biomass MW 30.90 30.90 0.00 Net power output of MSW MW 8.30 8.96 +0.66 Total energy efficiency % 27.40 27.86 +0.46 Waste-to-electricity efficiency % 20.49 22.12 +1.63

Several scholars have attempted to improve the WtE process by the integration with other power systems, such as a gas turbine combined cycle or coal-fired power plant. The performance of the current hybrid system was compared to the performances of two different hybrid systems in reference [26] and reference [28], as displayed in Table 10. By integrating the WtE system with a gas turbine combined cycle or coal-fired power plant, the waste-to-electricity efficacy can be promoted by 6.9 percentage points or 9.16 percentage points, both of which are larger than the efficiency improvement due to the incorporation with a biomass power plant in this paper. This is mainly because the steam parameters are relatively high in the combined cycle or coal power plant, and their steam cycles are more efficient. Whereas, the proposal in this paper provides another solution to enhance the WtE technology, especially when building a new WtE plant near an existing biomass-fired power plant. Furthermore, the current approach can achieve a hybrid system completely by using renewable energy sources (biomass and MSW), which can play an important role in addressing the issues of energy shortage and global warming.

Table 10. Performance comparison of the WtE systems integrated with various power systems.

Item Ref [26] Ref [28] Current paper WtE + gas turbine WtE + coal-fired WtE + biomass-fired Integrated scheme combined cycle power plant power plant Fuels MSW + MSW + coal MSW + biomass Waste-to-electricity efficiency of 26.9 20.49 20.49 conventional WtE system (%) Waste-to-electricity efficiency of 33.8 29.65 22.12 integrated WtE system (%) Improvement in waste-to-electricity 6.9 9.16 1.63 efficiency due to integration (%)

The detailed energy flows in the two schemes are displayed in Figure4. The feedstocks (biomass and MSW) provide the energy inputs of the two schemes, which are identical for the two schemes. In the separate scheme, the superheated steam of 42.61 MW from the WtE boiler goes into the turbine to produce work, and the extraction steam of 4.08 MW from the turbine is conveyed to warm the combustion air in the PAH2 and SAH. In the integrated scheme, 42.61-MW energy from the WtE boiler is Energies 2020, 13, x FOR PEER REVIEW 10 of 20

Table 10. Performance comparison of the WtE systems integrated with various power systems.

Item Ref [26] Ref [28] Current paper WtE + coal- WtE + biomass- WtE + gas turbine Integrated scheme fired power fired power combined cycle plant plant MSW + natural Fuels MSW + coal MSW + biomass gas Waste-to-electricity efficiency of 26.9 20.49 20.49 conventional WtE system (%) Waste-to-electricity efficiency of 33.8 29.65 22.12 integrated WtE system (%) Improvement in waste-to-electricity 6.9 9.16 1.63 efficiency due to integration (%)

The detailed energy flows in the two schemes are displayed in Figure 4. The feedstocks (biomass and MSW) provide the energy inputs of the two schemes, which are identical for the two schemes. In the separate scheme, the superheated steam of 42.61 MW from the WtE boiler goes into the turbine Energiesto produce2020, 13 work,, 4345 and the extraction steam of 4.08 MW from the turbine is conveyed to warm10 of the 20 combustion air in the PAH2 and SAH. In the integrated scheme, 42.61-MW energy from the WtE boiler is delivered to the turbine of the biomass power section for power generation. Meanwhile, the deliveredfeedwater to derived the turbine from of the the biomass biomass power power section section is for sent power to preheat generation. the air Meanwhile, into the WtE the boiler, feedwater then derivedflows into from the theWtE biomass boiler. Since power the section total steam is sent flow to rate preheat into the condensers air into the declines WtE boiler, by 0.83 then t/h flows after intothe incorporation, the WtE boiler. the Since overall the totalenergy steam loss flowof the rate condensers into the condensers decreases from declines 77.64 by MW 0.83 to t/ h77.16 after MW. the incorporation,Besides, the energy the overall losses energy of the loss turbines of the condensersand the decreases remain from unchanged 77.64 MW toin 77.16the two MW. schemes. Besides, theGenerally, energy the losses integrated of the turbines scheme generates and the boilers an additional remain power unchanged of 0.66 in MW the twocompared schemes. to the Generally, separate thescheme. integrated scheme generates an additional power of 0.66 MW compared to the separate scheme.

Water 6.72 MW (4.70%) Total energy efficiency：27.40% Steam 4.08 MW (2.85%) Waste-to-electricity efficiency：20.49%

Auxiliary power Steam 1.46 MW (1.02%) 42.61 MW (29.79%) Work MSW Boiler Turbine 9.86 MW (6.90%) Generator 40.51 MW (MSW) (MSW) (MSW) (28.32%) Net power Loss Loss Loss (Condenser) Loss (Turbine) 8.30 MW 0.10 MW 8.70 MW 21.93 MW 0.02 MW (5.80%) (6.08%) (15.33%) (0.01%) (0.07%)

Net total Sum: 20.79 MW Sum: 77.64 MW Sum: 0.09 MW Sum: 0.42 MW power (14.53%) (54.27%) (0.06%) (0.31%) 39.20 MW (27.40%)

Loss Loss (Condenser) Loss (Turbine) Loss Net power 12.09 MW Steam 55.71 MW 0.07 MW 0.35 MW 30.90 MW (8.45%) 122.10 MW (38.94%) (0.05%) (0.24%) (85.35%) (21.60%) Biomass 102.55 MW Boiler Turbine (71.68%) (Biomass) (Biomass) Generator (Biomass) Work 34.68 MW (24.24%) Auxiliary power Water 3.43 MW (2.40%) 31.65 MW (22.12%)

Energies 2020, 13, x FOR PEER REVIEW 11 of 20 (a)

Water 11.36 MW (7.94%)

Water Total energy efficiency：27.86% 0.56 MW (0.39%) Waste-to-electricity efficiency：22.12% MSW Boiler 40.51 MW (MSW) (28.32%)

Loss Steam 8.70 MW 42.61 MW (6.08%) (29.79%)

Sum: 20.79 MW Loss Loss (14.53%) (Condenser) (Turbine) Loss 77.16 MW 0.09 MW 0.45 MW (53.94%) (0.06%) (0.31%) Loss 12.09 MW (8.45%) Steam 122.10 MW (85.35%) Biomass 102.55 MW Boiler Turbines Net total (71.68%) (Biomass) (Biomass) Generator power (Biomass) 39.86 MW Work (27.86%) 45.01 MW (31.47%) Water 31.65 MW (22.12%) Auxiliary power 4.71 MW (3.29%)

(b)

Figure 4. Energy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme. Figure 4. Energy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme.

4.3. Exergy Analysis Exergy is regarded as the maximum useful work that can be utilized during the energy conversion process [30], and the exergy analysis can provide extra information as compared to the energy analysis [31]. EX The exergy of the feedstock ( f , kW) can be estimated as [32]:

=××+×+×+×www(H) (O) (N) EXf ( mff,net / 3.6) q (1.0064 0.1519 0.0616 0.0429 ) (4) www(C) (C) (C) m q where f is the feedstock (MSW or biomass) flow rate, t/h; f,net is the lower heating value of the w(H) w(C) w(O) w(N) feedstock, kJ/kg; and , , , and are the contents of hydrogen, carbon, oxygen, and nitrogen in the feedstock. η η The waste-to-electricity exergy efficiency ( ex,w ) and the total exergy efficiency ( ex,tot ) are formulated as: P η = w ex,w (5) EX w P η = tot ex,tot (6) EX+ EX b w EX EX where w and b are the exergy of the MSW and the exergy of the biomass, kW. The exergy flows of the two schemes were explored, as illustrated in Figure 5, where the exergy inflows, exergy outflows, and exergy losses of the main components are indicated. In the separate scheme, the MSW exergy (42.79 MW) and the biomass exergy (111.14 MW) enter into the WtE boiler and the biomass-fired boiler, and the boilers offer exergy (steam) to the steam cycles of the two reference plants, contributing a total exergy output (electricity) of 39.20 MW. In the integrated scheme, a portion of the feedwater is delivered from the DEA outlet into the PAH2 and SAH2 to warm the air, and then, the condensate enters into the WtE boiler; thereby, more steam extractions into RH3–6 will be needed for feedwater heating. Moreover, partial feedwater of the biomass power section is

Energies 2020, 13, 4345 11 of 20

4.3. Exergy Analysis Exergy is regarded as the maximum useful work that can be utilized during the energy conversion process [30], and the exergy analysis can provide extra information as compared to the energy analysis [31]. The exergy of the feedstock (EXf, kW) can be estimated as [32]:

w(H) w(O) w(N) EX = (m /3.6) q (1.0064 + 0.1519 + 0.0616 + 0.0429 ) (4) f f × f,net × × w(C) × w(C) × w(C) where mf is the feedstock (MSW or biomass) flow rate, t/h; qf,net is the lower heating value of the feedstock, kJ/kg; and w(H), w(C), w(O), and w(N) are the contents of hydrogen, carbon, oxygen, and nitrogen in the feedstock. The waste-to-electricity exergy efficiency (ηex,w) and the total exergy efficiency (ηex,tot) are formulated as: Pw ηex,w = (5) EXw

Ptot ηex,tot = (6) EXb + EXw where EXw and EXb are the exergy of the MSW and the exergy of the biomass, kW. The exergy flows of the two schemes were explored, as illustrated in Figure5, where the exergy inflows, exergy outflows, and exergy losses of the main components are indicated. In the separate scheme, the MSW exergy (42.79 MW) and the biomass exergy (111.14 MW) enter into the WtE boiler and the biomass-fired boiler, and the boilers offer exergy (steam) to the steam cycles of the two reference plants, contributing a total exergy output (electricity) of 39.20 MW. In the integrated scheme, a portion of the feedwater is delivered from the DEA outlet into the PAH2 and SAH2 to warm the air, and then, the condensate enters into the WtE boiler; thereby, more steam extractions into RH3–6 will be needed for feedwater heating. Moreover, partial feedwater of the biomass power section is used to preheat the air in the PAH1 and SAH2; thus, the steam extraction into the RH6 will be augmented. Attributed to the incorporation, the net total exergy output (electricity) is raised to 39.86 MW, which is 0.66 MW larger than that of the separate scheme. In addition, the total exergy loss in the boilers declines from 93.05 MW to 92.45 MW, which is mainly because the temperature differences in the PAHs and SAHs drop. The total exergy loss of the condensers dwindles by 0.42 MW due to the decrease of the steam into the condensers. The total exergy loss of the turbines is raised from 11.05 MW to 11.24 MW, and the total exergy loss of the RHs increases from 0.43 MW to 0.79 MW. The exergy losses of other components have no obvious changes. Therefore, the total exergy loss of the integrated scheme diminishes from 114.73 MW to 114.07 MW. Caused by the integration, the waste-to-electricity exergy efficiency is promoted from 19.40% to 20.94%, and the total exergy efficiency is enhanced from 25.46% to 25.89%. Energies 2020, 13, x FOR PEER REVIEW 12 of 20 used to preheat the air in the PAH1 and SAH2; thus, the steam extraction into the RH6 will be augmented. Attributed to the incorporation, the net total exergy output (electricity) is raised to 39.86 MW, which is 0.66 MW larger than that of the separate scheme. In addition, the total exergy loss in the boilers declines from 93.05 MW to 92.45 MW, which is mainly because the temperature differences in the PAHs and SAHs drop. The total exergy loss of the condensers dwindles by 0.42 MW due to the decrease of the steam into the condensers. The total exergy loss of the turbines is raised from 11.05 MW to 11.24 MW, and the total exergy loss of the RHs increases from 0.43 MW to 0.79 MW. The exergy losses of other components have no obvious changes. Therefore, the total exergy loss of the integrated scheme diminishes from 114.73 MW to 114.07 MW. Caused by the integration, the waste- Energiesto-electricity2020, 13 ,exergy 4345 efficiency is promoted from 19.40% to 20.94%, and the total exergy efficiency12 of 20is enhanced from 25.46% to 25.89%.

Loss Loss 0.24 MW (0.15%) 1.64 MW (1.07%)

Total exergy efficiency：25.46% Waste-to-electricity exergy efficiency ：19.40% Water 0.02 MW (0.01%) RHs Condenser (MSW) (MSW)

Steam Water Water Steam 1.66 MW 1.17 MW 0.10 MW Steam 1.29 MW (1.08%) Auxiliary power (0.76%) (0.06%) 1.59 MW (1.04%) (0.84%) 1.46 MW (0.95%)

Steam MSW 16.96 MW (11.01%) Generator Boiler Turbine 42.79 MW (MSW) (MSW) (MSW) Work (27.80%) 9.87 MW (6.41%) Loss Loss Loss 28.51 MW 2.54 MW 0.10 MW Net power (18.52%) (1.65%) (0.06%) 8.30 MW (5.39%)

Sum: 93.05 MW Sum: 11.05 MW Sum: 0.45 MW Net total (60.45%) (7.18%) (0.29%) power 39.20 MW (25.46%)

Loss Net power Loss Loss 64.54 MW 30.90 MW 8.51 MW 0.35 MW (41.93%) (20.07%) (5.53%) (0.23%) Work Biomass 34.68 MW Boiler 111.14 MW (22.53%) (Biomass) Turbine Generator (72.20%) (Biomass) (Biomass) Steam 54.76 MW (35.57%)

Steam Water 8.30 MW (5.39%) Steam 8.16 MW 3.27 MW (5.30%) (2.12%) Auxiliary power 3.43 MW (2.23%)

Water RHs 0.04 MW (0.03%) Condenser (Biomass) (Biomass)

Loss Loss 0.19 MW (0.13%) 3.23 MW (2.10%)

(a)

Figure 5. Cont.

EnergiesEnergies 20202020,, 1313,, x 4345 FOR PEER REVIEW 1313 of of 20 20

Loss 27.91 MW (18.13%)

Total exergy efficiency：25.89% Waste-to-electricity exergy efficiency：20.94%

Steam 16.96 MW (11.01%) MSW Boiler 42.79 MW (MSW) (27.80%)

Steam 10.95 MW (7.12%) Water Water 2.21 MW 0.14 MW (1.44%) (0.09%)

RHs Condenser (Biomass) Water (Biomass) 0.06 MW Steam (0.04%) 4.51 MW (2.93%)

Water Loss 8.16 MW Loss Auxiliary power 4.45 MW (2.89%) (5.30%) 0.79 MW (0.51%) 4.71 MW (3.06%)

Work Steam 45.01 MW 54.76 MW (35.57%) (29.24%) Biomass Net total Boiler Turbines Generator 111.14 MW power (Biomass) (Biomass) (Biomass) (72.20%) 39.86 MW (25.89%)

Loss Loss Loss 64.54 MW (41.93%) 11.24 MW (7.30%) 0.45 MW (0.29%)

(b)

Figure 5. Exergy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme. Figure 5. Exergy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme. 5. Economic Analysis 5. Economic Analysis An economic analysis, which considers both the cash inflows and cash outflows within the durationAn economic of a project, analysis, was conductedwhich considers to offer both another the viewcash inflows on the feasibilityand cash ofoutflows the novel within concept. the durationUnder the of conditiona project, thatwas theconduc expensested to and offer earnings another of view the biomasson the feasibility power section of the were novel deemed concept. as Underunchanged, the condition the WtE that section the expenses was solely and studied. earnings Table of the11 presentsbiomass thepower basic section economic were datadeemed for theas unchanged,evaluation. Thethe WtE reference section WtE was plant solely was studied. constructed Table in China11 presents with athe unit basic capital economic cost of 56,500data for USD the/t, evaluation.and the electricity The reference sale and WtE waste plant disposal was constructed fee are the in main China sources with a of unit cash capital inflows. cost of 56,500 USD/t, and theDue electricity to the novel sale solution,and waste some disposal components fee are the in themain WtE sources section of andcashthe inflows. biomass power section are removed, retrofitted, or added. The capital costs of the changed equipment were calculated by the methods shown in Table Table12, and 11. theBasic results data for are the given economic in Table analysis. 13. Several components (turbine, condenser, stack, etc.) of the WtE plantItem are removed after the integration; thereby,Unit the capitalValue costs of 6649.68 thousandCapital USD can cost be of saved. reference As someWtE plant extra [33] components (PRV and million AP) USD are installed 28.25 for the hybrid configuration, an additional capital cost of 0.83 thousand USD is needed. Moreover, the PAH Operational cost of reference WtE plant [33] million USD 2.83 and SAH of the WtE boiler and the turbine and generator of the biomass power section are remolded for the incorporation; the relative capital cost will augmentConstruction by 4407.10 thousandyear USD. Consequently, 2 Lifetime of WtE plant [34] the total capital cost of the WtE section diminishes by 2241.74Economic thousand USDyear due to the new23 design. Annual operating time of WtE plant [34] h 7200 Loan ratio [33] - 70% Loan term [33] year 15 Interest rate [33] - 6.15% Discount rate [33] - 12%

Energies 2020, 13, 4345 14 of 20

Table 11. Basic data for the economic analysis.

Item Unit Value Capital cost of reference WtE plant [33] million USD 28.25 Operational cost of reference WtE plant [33] million USD 2.83 Construction year 2 Lifetime of WtE plant [34] Economic year 23 Annual operating time of WtE plant [34] h 7200 Loan ratio [33] - 70% Loan term [33] year 15 Interest rate [33] - 6.15% Discount rate [33] - 12% Disposal fee [33] USD/t 9.65 Feed-in tariff [35] USD/(kW h) 0.97 · 1st–3rd year - 0% Income tax rate in economic duration [34] 4th–6th year - 12.5% 7th–23rd year - 25.0%

Table 12. Capital cost estimation methods for the changed equipment.

Scaling Up Method Basic Cost Component Basic Scale Scale Unit Scale Factor Reference (Million USD) PAH, SAH 0.78 8372 m2 1 [36,37] Stack 10.65 4039.2 t/h 1 Condenser 4.28 36000 m2 1 [36,38] Cooling tower 17.01 13000 m2 1 Function method 0.71 Turbine CCT= 6000 (WT,nom) × [39] Pump CC = 3540 (W )0.71 P × P,nom Generator CC = 60 (P )0.95 [40] G × G,nom DEA CC = 6014 (m )0.7 [28] DEA × fw h i2 RH log (CC ) = 4.8306 0.8509 log (A) + 0.3187 log (A) [41] 10 RH − × 10 × 10 0.68 PRV CC = 37 (p /pout) [42] PRV × in Note: CCT, CCP, CCG, CCDEA, CCRH, and CCPRV are the capital costs of the turbine, pump, generator, DEA, RH, and pressure reducing valve (PRV), USD; WT,nom is the nominal work output of the turbine, kW; WP,nom is the nominal work consumption of the pump, kW; PG,nom is the nominal power output of the generator, kW; mfw is 2 the feedwater flow rate, kg/s; A is the area, m ; and pin and pout are the inlet pressure and outlet pressure, kPa. Energies 2020, 13, 4345 15 of 20

Table 13. Capital costs of the changed equipment. CP: condensate pump and AP: additional pump.

Separate Scheme Integrated Scheme Component Difference (Thousand USD) (Thousand USD) Turbine 4111.07 - 4111.07 − Generator 370.30 - 370.30 − Condenser 118.78 - 118.78 − Circulating water pump 146.65 - 146.65 − Cooling tower 1504.68 - 1504.68 − WtE section CP 9.02 - 9.02 − RH 19.08 - 19.08 − DEA 32.30 - 32.30 − Stack 337.81 - 337.81 − PAH 178.94 297.13 +118.19 SAH 42.14 119.01 +76.87 Turbine 10036.51 13904.86 +3868.34 Biomass power section Generator 1221.93 1565.63 +343.70 PRV - 0.11 +0.11 AP - 0.72 +0.72 Sum 18129.20 15887.46 2241.74 −

To examine the economic feasibility, two indicators have been adopted, the dynamic payback period (DPP, year) and the net present value (NPV, USD) [43]:

DPP X C Cout in − = 0 (7) ( + r )y y=1 1 dis

b X C Cout NPV = in − (8) ( + r )y y=1 1 dis where Cin is the cash inflow, USD, Cout is the cash outflow, USD, y is the year number in the plant lifetime, b is the plant lifetime, year, and rdis is the discount rate. As shown in Table 14, the total capital cost of the WtE section is cut down by 2.24 million USD due to the proposal; meanwhile, the operational cost decreases from 2.83 million USD to 2.60 million USD. Since the feedstock (MSW) flow rate is invariable, the MSW disposal capacities in the two schemes are both 150,000 t, and the waste disposal fee remains fixed. Induced by the hybridization, the electricity supply is improved from 59.76 GW h to 64.52 GW h; thereby, the revenue of electricity sales increase · · by 0.46 million USD. Hence, the dynamic payback period of the WtE section declines from 8.18 years to 5.37 years, and the net present value grows from 5.26 million USD to 10.28 million USD. Energies 2020, 13, 4345 16 of 20

Table 14. Economic analysis results of the WtE section in the two schemes.

Item Separate Scheme Integrated Scheme Difference Total capital cost (million USD) 28.25 26.01 2.24 − Operational cost (million USD) 2.83 2.60 0.23 − Annual amount of MSW disposal (103 t) 150 150 0.00 Annual electricity supply (GW h) 59.76 64.52 +4.76 · Annual revenue due to MSW disposal (million USD) 1.45 1.45 0.00 Annual revenue due to electricity sale (million USD) 5.77 6.23 +0.46 Total annual revenue (million USD) 7.22 7.68 +0.46 Net present value (million USD) 5.26 10.28 +5.02 Dynamic payback period (year) 8.18 5.37 2.81 −

6. Sensitivity Analysis Regarding the operation of the hybrid system, the dominating factors are the conditions of the biomass-fired boiler and the WtE boiler. The loads of the two boilers basically impact the live steam parameters; thereby, the performance of the steam cycle is mostly determined. The boiler loads are mainly dependent on the fuel qualities and quantities. Normally, while the fuel qualities/types of the two boilers change in limited ranges, the boiler loads can be adjusted by regulating the fuel feed rates. On the condition that the boiler loads are constant, the live steam parameters of the two boilers can be nearly maintained identical. Hence, a sensitivity analysis has been undertaken to check how the boiler loads affect the performance of the hybrid system. However, the effects of the fuel types or qualities were not discussed. Besides, the fuel types or qualities are necessary to remain relatively stable for the two boilers during operation. If the fuel types or qualities are far from the design ones, the two boilers cannot work well or may even shut down, especially the WtE boiler. Figure6 displays the influence of the biomass-fired boiler load on the performance of the hybrid system. It can be seen that the waste-to-electricity efficiency decreases when the biomass-fired boiler load declines from 100% to 90%, but then, the waste-to-electricity efficiency grows along with the decrement of the load. This is chiefly because the temperature of the feedwater that the combustion air of the WtE boiler in the PAH2 and SAH2 diminishes with the descending biomass-fired boiler load, and the feedwater from the RH2 outlet is employed for air preheating once the biomass-fired boiler load ranges from 60% to 90%. As a result, the power consumption of the FWP1 increases, which reduces the net power output of the MSW. Besides, with the decrease of the biomass-fired boiler load condition (from 90% to 60%), the outlet air temperatures of the PAH1/SAH1will be higher; thereby, more low-grade heat from the biomass-fired section can be used to warm the air instead of the high-grade heat, which is favorable to producing more power. The performance of the hybrid system varying with the WtE boiler load is presented in Figure7. With the decrease of the WtE boiler load, the DEA inlet steam pressure and the feedwater temperature into the PAH2/SAH2 dwindle. Furthermore, the air temperature between the PAH1 and PAH2 (or SAH1 and SAH2) gets larger, leading to utilizing more low-grade heat from the biomass power section. As a consequence, while the WtE boiler load falls, the net power output of the MSW drops, but the waste-to-electricity efficiency rises slightly. Energies 2020, 13, x FOR PEER REVIEW 17 of 20

9.4 22.5 Net power output of MSW Waste-to-electricity efficiency 9.2 22.0

Energies 2020, 13, 4345 9.0 17 of 20 Energies 2020, 13, x FOR PEER REVIEW 17 of 20 21.5

8.89.4 22.5 Net power output of MSW Waste-to-electricity efficiency 21.0 9.2

Net power output of MSW (MW)of output power Net 8.6 22.0 (%) efficiency Waste-to-electricity

8.49.0 20.5 60% 70% 80% 90% 100% 21.5 Load of biomass-fired boiler

8.8 Figure 6. Effect of the biomass-fired boiler load on the performance of the integrated WtE system.

21.0

The performance MSW (MW)of output power Net of8.6 the hybrid system varying with the WtE boiler load is presented in Figure Waste-to-electricity efficiency (%) efficiency Waste-to-electricity 7. With the decrease of the WtE boiler load, the DEA inlet steam pressure and the feedwater temperature into the PAH2/SAH2 dwindle. Furthermore, the air temperature between the PAH1 and 8.4 20.5 PAH2 (or SAH1 and SAH2)60% gets larger, 70% leading 80%to utilizing 90% more low-grade 100% heat from the biomass power section. As a consequence, whileLoad the of WtE biomass-fired boiler loadboiler falls, the net power output of the MSW drops, but the waste-to-electricity efficiency rises slightly. Figure 6. EffectEffect of the biomass-fired biomass-fired boiler load on thethe performance of the integrated WtE system.

9.5 23.0 The performance of the hybrid Netsystem power varyingoutput of MSW with the WtE boiler load is presented in Figure 9.0 7. With the decrease of the WtE boiler Waste-to-electricity load, the efficiency DEA inlet steam pressure22.8 and the feedwater temperature into the PAH2/SAH28.5 dwindle. Furthermore, the air temperature between the PAH1 and PAH2 (or SAH1 and SAH2) gets larger, leading to utilizing more low-grade22.6 heat from the biomass power section. As a consequence,8.0 while the WtE boiler load falls, the net power output of the MSW drops, but the waste-to-electricity7.5 efficiency rises slightly. 22.4

7.0 9.5 23.022.2

6.5 Net power output of MSW 9.0 Waste-to-electricity efficiency 22.822.0 6.0

Net power output of MSW (MW) of output powerNet 8.5 Waste-to-electricity efficiency (%) efficiency Waste-to-electricity 22.621.8 5.5 8.0

5.0 21.6 7.5 22.4 60% 70% 80% 90% 100% Load of WtE boiler 7.0 22.2 Figure 7. Effect of the WtE boiler load on the performance of the integrated WtE system. Figure 7. Effect6.5 of the WtE boiler load on the performance of the integrated WtE system. 7. Conclusions 22.0 6.0

7. Conclusions MSW (MW) of output powerNet Waste-to-electricity efficiency (%) efficiency Waste-to-electricity For improving the energy utilization efficiency of the waste-to-energy process,21.8 this paper developed 5.5 an innovativeFor improving design the that energy combines utilization a waste-to-energy efficiency plantof the with waste-to-energy a biomass-fired process, power plantthis basedpaper ondeveloped the steam an cycle innovative integration.5.0 design According that combines to the a novelwaste-to-energy concept, the plant superheated with21.6 a biomass-fired steam generated power by plant based on the steam cycle60% integration. 70% Accord 80%ing to the 90% novel concept, 100% the superheated steam the waste-to-energy boiler enters into the turbineLoad of WtE of the boiler biomass power section for power production. Meanwhile,generated by the the air waste-to-energy fed into the waste-to-energy boiler enters boilerinto the is warmed turbine byof thethe feedwaterbiomass power from the section biomass for power production. Meanwhile, the air fed into the waste-to-energy boiler is warmed by the feedwater power sectionFigure and 7. theEffect saturated of the WtE steam boiler from load the on waste-to-energythe performance of boiler, the integrated and the biomassWtE system. power section suppliesfrom the feedwaterbiomass power for the section waste-to-energy and the saturated boiler. To steam assess from the newthe waste-to-energy design thermodynamically boiler, and andthe economically,7.biomass Conclusions power the section separate supplies schemes feedwater and the for integrated the waste-to-energy scheme were boiler. simulated To assess based the on new a 35-MWdesign biomass-fired power plant and a 500-t/d waste-to-energy plant. When the feedstock (biomass and For improving the energy utilization efficiency of the waste-to-energy process, this paper municipal solid waste) flow rates are constant, the net power output of municipal solid waste developed an innovative design that combines a waste-to-energy plant with a biomass-fired power is promoted by 0.66 MW, and the waste-to-electricity efficiency increases from 20.49% to 22.12%. plant based on the steam cycle integration. According to the novel concept, the superheated steam Energy and exergy analyses were implemented to uncover the root cause of efficiency-boosting. generated by the waste-to-energy boiler enters into the turbine of the biomass power section for The decrease of steam into the condensers is the main cause of energy loss reduction. On the other power production. Meanwhile, the air fed into the waste-to-energy boiler is warmed by the feedwater hand, the exergy losses of the boilers and the condensers are diminished significantly, induced by the from the biomass power section and the saturated steam from the waste-to-energy boiler, and the biomass power section supplies feedwater for the waste-to-energy boiler. To assess the new design

Energies 2020, 13, 4345 18 of 20 decreased temperature differences of the air preheaters and the decline of the steam into the condensers. The economic analysis results indicate that the dynamic payback period of the waste-to-energy section diminishes from 8.18 years to 5.37 years, and the net present value is raised by 5.02 million USD. Besides, the performance of the new design was investigated under various boiler loads. In summary, the proposal is extremely feasible from the viewpoints of and economics. The current research has been one of the first attempts to integrate the energy conversions of municipal solid waste and biomass, which lays the groundwork for future research.

Author Contributions: Conceptualization, P.P. and H.C.; data curation, M.Z., H.C. and X.S.; formal analysis, P.P. and M.Z.; funding acquisition, G.X. and H.C.; investigation, P.P., M.Z. and X.S.; software, M.Z.; supervision, G.X. and T.L.; writing—original draft, P.P. and M.Z.; and writing—review and editing, G.X. and H.C. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Science Fund of China (No. 51806062) and the Fundamental Research Funds for the Central Universities (No. 2020MS006). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Ning, C.; You, F. Data-driven Wasserstein distributionally robust optimization for biomass with agricultural waste-to-energy network design under uncertainty. Appl. Energy 2019, 255, 113857. [CrossRef] 2. Martínez-Guido, S.I.; Ríos-Badrán, I.M.; Gutiérrez-Antonio, C.; Ponce-Ortega, J.M. Strategic planning for the use of waste biomass pellets in Mexican power plants. Renew. Energy 2019, 130, 622–632. [CrossRef] 3. Bermúdez, C.A.; Porteiro, J.; Varela, L.G.; Chapela, S.; Patiño, D. Three-dimensional CFD simulation of a large-scale grate-fired biomass furnace. Fuel Process. Technol. 2020, 198, 106219. [CrossRef] 4. Situmorang, Y.A.; Zhao, Z.; Yoshida, A.; Abudula, A.; Guan, G. Small-scale biomass gasification systems for power generation (<200 kW class): A review. Renew. Sustain. Energy Rev. 2020, 117, 109486. 5. Osman, A.I.; Abdelkader, A.; Farrell, C.; Rooney, D.; Morgan, K. Reusing, recycling and up-cycling of biomass: A review of practical and kinetic modelling approaches. Fuel Process. Technol. 2019, 192, 179–202. [CrossRef] 6. Nunes, L.J.R.; Causer, T.P.; Ciolkosz, D. Biomass for energy: A review on supply chain management models. Renew. Sustain. Energy Rev. 2020, 120, 109658. [CrossRef] 7. Zhu, Y.; Liang, J.; Yang, Q.; Zhou, H.; Peng, K. Water use of a biomass direct-combustion power generation system in China: A combination of life cycle assessment and water footprint analysis. Renew. Sustain. Energy Rev. 2019, 115, 109396. [CrossRef] 8. Liu, D.; Liu, M.; Xiao, B.; Guo, X.; Niu, D.; Qin, G.; Jia, H. Exploring biomass power generation’s development under encouraged policies in China. J. Clean. Prod. 2020, 258, 120786. [CrossRef] 9. Yuan, J.; Luo, X.; Ding, X.; Liu, C.; Li, C. Biomass power generation fuel procurement and storage modes evaluation: A case study in Jilin. Renew. Sustain. Energy Rev. 2019, 111, 75–86. [CrossRef] 10. Mandø, M. Direct combustion of biomass. In Biomass Combustion Science, Technology and ; Rosendahl, L., Ed.; Woodhead Publishing, 2013. Available online: https://www.sciencedirect.com/science/ article/pii/B9780857091314500042 (accessed on 21 August 2020). 11. Vamvuka, D.; Loukakou, E.; Sfakiotakis, S.; Petrakis, E. The impact of a combined pre-treatment on the combustion performance of various biomass wastes and their blends with lignite. Acta 2020, 688, 178599. [CrossRef] 12. Chen, G.; Wang, X.; Li, J.; Yan, B.; Wang, Y.; Wu, X.; Velichkova, R.; Cheng, Z.; Ma, W. Environmental, energy, and economic analysis of integrated treatment of municipal solid waste and sewage sludge: A case study in China. Sci. Total Environ. 2019, 647, 1433–1443. [CrossRef][PubMed] 13. Guo, Y.; Glad, T.; Zhong, Z.; He, R.; Tian, J.; Chen, L. Environmental life-cycle assessment of municipal solid waste incineration stocks in Chinese industrial parks. Resour. Conserv. Recycl. 2018, 139, 387–395. [CrossRef] 14. Singh, K.; Hachem-Vermette, C. Influence of mixed-use neighborhood developments on the performance of waste-to-energy CHP plant. Energy 2019, 189, 116172. [CrossRef] 15. Lombardi, L.; Carnevale, E.; Corti, A. A review of and performances of thermal treatment systems for energy recovery from waste. Waste Manag. 2015, 37, 26–44. [CrossRef] Energies 2020, 13, 4345 19 of 20

16. Zhang, H.; Yu, S.; Shao, L.; He, P. Estimating source strengths of HCl and SO2 emissions in the flue gas from waste incineration. J. Environ. Sci.-China 2019, 75, 370–377. [CrossRef] 17. Xu, H.; Lin, W.Y.; Dal Magro, F.; Li, T.; Py, X.; Romagnoli, A. Towards higher energy efficiency in future waste-to-energy plants with novel storage-based thermal buffer system. Renew. Sustain. Energy Rev. 2019, 112, 324–337. [CrossRef] 18. Bogale, W.; Viganò, F.A Preliminary Comparative Performance Evaluation of Highly Efficient Waste-to-Energy Plants. Energy Procedia 2014, 45, 1315–1324. [CrossRef] 19. Martin, J.J.E.; Koralewska, R.; Wohlleben, A. Advanced solutions in combustion-based WtE technologies. Waste Manag. 2015, 37, 147–156. [CrossRef] 20. Liuzzo, G.; Verdone, N.; Bravi, M. The benefits of flue gas recirculation in waste incineration. Waste Manag. 2007, 27, 106–116. [CrossRef] 21. Strobel, R.; Waldner, M.H.; Gablinger, H. Highly efficient combustion with low excess air in a modern energy-from-waste (EfW) plant. Waste Manag. 2018, 73, 301–306. [CrossRef] 22. Main, A.; Maghon, T. Concepts and experiences for higher plant efficiency with modern advanced boiler and incineration technology. In Proceedings of the 18th Annual North American Waste-to-Energy Conference, Orlando, FL, USA, 11–13 May 2010. 23. Hulgaard, T.; MSc, I.S. Integrating waste-to-energy in Copenhagen, Denmark. Proc. Inst. Civ. Eng.-Civ. Eng. 2018, 171, 3–10. [CrossRef] 24. Behzadi, A.; Gholamian, E.; Houshfar, E.; Habibollahzade, A. Multi-objective optimization and exergoeconomic analysis of waste heat recovery from Tehran’s waste-to-energy plant integrated with an ORC unit. Energy 2018, 160, 1055–1068. [CrossRef] 25. Consonni, S.; Silva, P.Off-design performance of integrated waste-to-energy, combined cycle plants. Appl. Eng. 2007, 27, 712–721. [CrossRef] 26. Poma, C.; Verda, V.; Consonni, S. Design and performance evaluation of a waste-to-energy plant integrated with a combined cycle. Energy 2010, 35, 786–793. [CrossRef] 27. Mendecka, B.; Lombardi, L.; Gładysz, P.; Stanek, W. Exergo-Ecological Assessment of Waste to Energy Plants Supported by Solar Energy. Energies 2018, 11, 773. [CrossRef] 28. Chen, H.; Zhang, M.; Xue, K.; Xu, G.; Yang, Y.; Wang, Z.; Liu, W.; Liu, T. An innovative waste-to- integrated with a coal-fired power plant. Energy 2020, 194, 116893. [CrossRef] 29. STEAG Energy Services, EBSILON Professional Documentation; 2020. Available online: https://www.ebsilon. com/en/ (accessed on 21 August 2020). 30. Liu, Z.; Liu, B.; Guo, J.; Xin, X.; Yang, X. Conventional and advanced exergy analysis of a novel transcritical compressed carbon dioxide system. Energy Convers. Manag. 2019, 198, 111807. [CrossRef] 31. Mahabadipour, H.; Srinivasan, K.K.; Krishnan, S.R. An exergy analysis methodology for internal combustion using a multi-zone simulation of dual fuel low temperature combustion. Appl. Energy 2019, 256, 113952. [CrossRef] 32. Zhou, J.; Ling, P.; Su, S.; Xu, J.; Xu, K.; Wang, Y.; Hu, S.; Zhu, M.; Xiang, J. Exergy analysis of a 1000 MW single reheat advanced supercritical carbon dioxide coal-fired partial flow power plant. Fuel 2019, 255, 115777. [CrossRef] 33. Zhao, X.; Jiang, G.; Li, A.; Wang, L. Economic analysis of waste-to-energy industry in China. Waste Manag. 2016, 48, 604–618. [CrossRef] 34. Xiangba, Z. Cost analysis of waste incineration-power generation projects. Environ. Sanit. Eng. 2014, 22, 57–60. (In Chinese) 35. National Development and Reform-Commission. The Notice on Improving Price Policies of WtE Plants; Beijing, China, 2012. (In Chinese). Available online: http://www.nea.gov.cn/2014-09/29/c_133682178.htm (accessed on 21 August 2020). 36. Ye, C. Research on the Key Issue of Coal Partial Gasification Technology; Zhejiang University: Hangzhou, China, 2018. (In Chinese) 37. Han, Y. Collaborative Optimization of Energy Utilization and Pollutant Emission Reduction in Cold End of Boiler in Coal-Fired Power Generation System; North China Electric Power University: Beijing, China, 2018. (In Chinese) 38. Electric Power Planning and Engineering-Institute. Quota Design Reference Cost Indicators for Thermal Power Projects (2017 Level); China Electric Power Press: Beijing, China, 2018. (In Chinese) Energies 2020, 13, 4345 20 of 20

39. Ogorure, O.J.; Oko, C.O.C.; Diemuodeke, E.O.; Owebor, K. Energy, exergy, environmental and economic analysis of an agricultural waste-to-energy integrated multigeneration thermal power plant. Energy Convers. Manag. 2018, 171, 222–240. [CrossRef] 40. Lian, Z.T.; Chua, K.J.; Chou, S.K. A thermoeconomic analysis of biomass energy for trigeneration. Appl. Energy 2010, 87, 84–95. [CrossRef] 41. Zhang, C.; Liu, C.; Wang, S.; Xu, X.; Li, Q. Thermo-economic comparison of subcritical organic Rankine cycle based on different heat exchanger configurations. Energy 2017, 123, 728–741. [CrossRef] 42. Owebor, K.; Oko, C.O.C.; Diemuodeke, E.O.; Ogorure, O.J. Thermo-environmental and economic analysis of an integrated municipal waste-to-energy solid oxide , gas-, steam-, organic fluid- and absorption refrigeration cycle thermal power plants. Appl. Energy 2019, 239, 1385–1401. [CrossRef] 43. Liu, M.; Zhang, X.; Ma, Y.; Yan, J. Thermo-economic analyses on a new conceptual system of waste heat recovery integrated with an S-CO2 cycle for coal-fired power plants. Energy Convers. Manag. 2018, 161, 243–253. [CrossRef]

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